I recommend a vest size no larger two-thirds (2/3) of your pups girth measurement.
As a rule... I do not recommend sizing up since that will mean more fabric is under the dog than necessary.
I leave the straps very long intentionally on vests I make. This allows vests to be adjusted to fit larger dogs. The panel may be small looking on the larger pup, but it will still be able to be worn - making it a long lasting piece of gear.
To find your max recommended vest size...
- Measure your pups girth which is dimension C on the picture below.
2. Take the girth measurement and multiply it by 2/3. This will give you the largest size I recommend.
Example:
Measured a girth of 30 inches.
Max vest recommended = 30 * 2 / 3 = 20 inch vest recommended
Example:
Measured a girth of 20 inches.
Max vest recommended = 20 * 2 / 3 = 13 inch vest recommended
Example:
Measured a girth of 28 inches.
Max vest recommended = 28 * 2 / 3 = 18 inch vest recommended
Below is a table that you can reference rather than doing math.